This invention relates to fuel cells and, in particular, to a fuel reforming unit for use with such fuel cells.
A fuel cell is a device, which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Fuel cells operate by passing a reactant fuel gas through the anode, while passing oxidizing gas through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Before undergoing the electrochemical reaction in the fuel cell, hydrocarbon fuels such as methane, coal gas, etc. are typically reformed to produce hydrogen for use in the anode of the fuel cell. In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels without the need for expensive and complex reforming equipment. In addition, the endothermic reforming reaction can be used advantageously to help cool the fuel cell stack.
Internally reforming fuel cells employing direct internal reforming and indirect internal reforming have been developed. Direct internal reforming is accomplished by placing the reforming catalyst within the active anode compartment. The major disadvantage of direct internal reforming is the exposure of the catalyst to the electrolyte of the fuel cell, which can significantly degrade the fuel cell's performance. Improvements to the direct internal reforming technique intended to avoid electrolyte contamination have suffered from other disadvantages such as the high cost due to the complexity of fuel cell design, special materials requirements and reduction in the effectiveness of the reforming catalyst.
The second reforming technique, indirect internal reforming, is accomplished by placing the reforming catalyst in an isolated chamber within the fuel cell stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. The disadvantages of indirect internal reforming, however, include the requirement of separate ducting systems, the high cost of the fuel cell stack and the susceptibility to fuel leaks.
The present state of the art utilizes a hybrid assembly of a fuel cell with both direct and indirect internal reforming. U.S. Pat. No. 6,200,696 describes such a hybrid assembly, in which the indirect internal reformer is designed with a substantially U-shaped flow geometry, which allows the inlet fuel feed tubes to also be contained within the fuel-turn manifold thereby mitigating the risk of system fuel leaks. This configuration, however, results in large temperature gradients near the edge of the fuel cell plate due to a non-optimized flow field and catalyst distribution. In another assembly, U.S. patent application Ser. No. 10/269,481, assigned to the same assignee hereof, an improved reformer is provided which has a substantially U-shaped flow geometry and includes a plate assembly for supporting a reforming catalyst and a compliant baffle cooperating with the plate assembly to provide an improved flow field. The baffle and plate assembly of the '481 application segment the enclosure into several sections, including an inlet section, a turn section, a return section and an outlet section. The compliant baffle of the '481 application allows for improved sealing to prevent the escape of gases from the reformer and is arranged to direct the flow of gas to predetermined areas of the plate. Additionally, the reforming catalyst in the reformer of the '481 application is disposed in a pattern such that the reformer is devoid of catalyst in the inlet section to a point in the turn section and includes catalyst from that point in the turn section through the return section, varying in amount in a predetermined manner.
The aforesaid assembly of the '481 patent application provides an improved flow field and catalyst distribution. As a result, non-uniformity in reforming and temperature gradients in the assembly are reduced by counteracting a natural temperature distribution within the assembly and by providing more cooling to the hottest areas of the plate. However, because electrochemical heating and reforming cooling are very complex interactions, the hottest areas of the plate are not always the areas which require the most cooling as provided in the reformer of the '481 application. For example, the location of the fuel cell's maximum temperature as well as its current density, need to be considered in providing increased fuel flow and cooling to predetermined areas of the plate. Moreover, temperature distribution in the assembly at different times during the operating life also should be taken into account when regulating temperature distribution within the assembly.
It is therefore an object of the present invention to provide further improvements in the fuel flow field and catalyst distribution in the reformer assembly.
It is a further object of the present invention to provide a reformer assembly, which has enhanced performance and endurance.